1. Field of the Invention
The present invention relates to an optical phase modulator of the optical fiber type, for use with a fiber optic sensor such as a fiber optic gyroscope for measuring changes in various physical quantities including angular displacement, velocity, and acceleration.
2. Description of the Prior Art
Heretofore, optical phase modulators are generally classified into waveguide-type optical phase modulators and optical-fiber-type optical phase modulators. The waveguide-type optical phase modulators comprise an optical waveguide and electrodes which are mounted on a dielectric substrate of lithium niobate. The optical-fiber-type optical phase modulators comprise a cylindrical piezoelectric vibrator and an optical fiber wound therearound. The optical-fiber-type optical phase modulators can easily be optically coupled with a fiber optic sensor such as a fiber optic gyroscope by an optical fiber, a feature which is not possible with the waveguide-type optical phase modulators. The optical-fiber-type optical phase modulators are also advantageous in that they are simple in structure and can easily be manufactured.
FIG. 1 of the accompanying drawings shows a conventional optical phase modulator of the optical fiber type, generally designated by the reference numeral 100. The optical phase modulator 100 includes a cylindrical piezoelectric vibrator 101 which is vibratable in the diametric direction. The optical phase modulator 100 also has an optical fiber 102 wound tightly in several turns around the cylindrical piezoelectric vibrator 101. A light beam to be modulated in phase is transmitted into the optical fiber 102 from one end thereof, and a phase-modulated light beam leaves from the other end of the optical fiber 102. Disc-shaped electrodes 103, 104 are mounted respectively on the axial ends of the cylindrical piezoelectric vibrator 101. When a modulating signal voltage is applied between the electrodes 103, 104, the piezoelectric vibrator 101 is diametrically vibrated, inducing longitudinal stresses and strains in the optical fiber 102 wound around the piezoelectric vibrator 101. The effective refractive index and length of the optical fiber 102 slightly change due to the induced stresses and strains thereof. Therefore, the time needed for the light beam to pass through the optical phase modulator varies depending on the applied modulating signal voltage, impressing phase modulation on the light beam.
The optical-fiber-type optical phase modulators are however disadvantageous in that the phase shift amplitude vs. modulating frequency characteristic curve is irregular because of several different resonant frequencies in a vibration mode depending on the configuration of the optical-fiber-type optical phase modulators. For example, FIG. 2 of the accompanying drawings shows the relationship between the frequencies of a modulating signal, whose voltage is constant, applied to the piezoelectric vibrator 101 and the amplitudes of phase shifts of a light beam that passes through the optical phase modulator 100. The phase shift amplitude vs. modulating frequency characteristic curve shown in FIG. 4 is characterized by irregularities or resonant peaks in a vibration mode of the piezoelectric vibrator 101. The irregular characteristic curve makes it impossible to modulate the phase of a light beam with a modulating signal having a desired waveform for highly accurate measurements with sophisticated signal processing.